In a high frequency signal line for use in an MEMS device or the like, an insulating layer is generally formed along a part of a semiconductor layer having the device formed thereon (i.e., a region serving as a signal line), and then, a strip conductor is mounted on the insulating layer. As a consequence, the width of such a semiconductor layer is sufficiently greater than the line width of the strip conductor, and therefore, the semiconductor layer uses a substrate having a larger area.
However, when, for example, two signal lines for a common semiconductor layer are wired in parallel to each other on the common semiconductor layer in the case where the signal lines are wired in proximity with and in parallel to each other, electromagnetic coupling occurs between the two signal lines through the semiconductor layer. When the electromagnetic coupling occurs between the two signal lines, their high frequency transmission signals are mixed to generate noise or a crosstalk, or may be leaked, thereby resulting in larger transmission loss. In addition, since an electromagnetic field generated around the signal lines is enlarged inside of the semiconductor layer, electromagnetic coupling may occur between the signal line and another device, which is arranged in the vicinity of the signal line. The occurrence of such electromagnetic coupling induces a leak of a high frequency signal that is transmitted through the signal line, thereby increasing a transmission loss.
Patent Document 1 discloses a method for reducing the above-described transmission loss in a signal line. As shown in FIG. 1, a microstrip line is formed by mouting a strip conductor 13 on a dielectric board 12 having a ground conductor 11 formed at the reverse thereof, and further, grooves 14 are formed in the dielectric board 12 along and near both side ends of the lower surface of the strip conductor 13.
An electric field is concentrated at both side ends of the lower surface of the strip conductor 13 in a microstrip line not having any grooves 14. In view of this, the grooves 14 are formed in the vicinity of both side ends of the lower surface of the strip conductor 13 in the microstrip line disclosed in Patent Document 1, therefore, a medium in the vicinity of both side ends of the lower surface of the strip conductor 13 is changed from a dielectric material (the dielectric board 12) to air having a permittivity of 1. In this manner, the concentration of the electric field is alleviated in the vicinity of both side ends of the lower surface of the strip conductor 13, so that a transmission loss is reduced.
However, in the case where metal wiring is formed on the insulating layer (an insulating coating film) formed on the surface of the semiconductor layer, no effect can be expected by forming the grooves on the insulating layer since the insulating layer is very thin. Therefore, the structure disclosed in Patent Document 1 in which the grooves are formed on the dielectric board provided with the strip conductor thereon cannot be adopted.
Patent Document 1: Japanese Unexamined Patent Publication No. 09-246814